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Full text of "The Cambridge Natural History"

NUMBER OF TOES

43

the   hind-limbs   must   have    preceded   the   fore-limbs   in   their

thorough adaptation  to  the  cnrsorial   mode of progression.      In

the  Mammalia   the   ankle-joint   is   always

what   is   termed   cruro-tarsal,  i.e.   between

the ends of the limb-bones and the proximal

row   of  tar sals;   not  in  the  middle of  the

tarsus as in  some   Sauropsida (reptiles arid

birds).      The bones of the  ankle are much

like   those   of   the   hand ;    but   there    are

never   more   than two bones   in   the   proxi-   tr

mal row, which are the  astragalus and the

calcaneum.      The   former  is   perhaps  to be

looked    upon    as    the     equivalent    of   the

cuneiform   and   lunar   together.      Kut   the

views  as  to   the   homologies  of the  tarsai

bones   differ   widely.      IRelow   these   is  the

navicular,   regarded   as   a   centrale.       The

distal   row   of   the   tarsus   has   four   bones,

three cuneiforms and a cuboid.     Reduction   FJGL 32.—Anterior aspect of

is   effected   by  the   soldering    together   of      right femur of Rhinoceros

,                    . rt                            ,-,        -r-f            i         ,-,           (Rhinoceros indicus").   x i.

two   cuneiiorms   as   in   the  Horse,   by   the       -

fusion  of  the   navicular   and cuboid  as   in

the Deer.     2STo mammal has more than five

toes, and the number tends to become reduced in cursorial animals

(Rodents, Ungulates, Kangaroos).

Teeth.---The   teeth of the  Mammaliax   differ  from  those  of

other vertebrated animals in a number of important points.
These, however, entirely concern the form of the adult teeth,
their position in the mouth, and the succession of the series of
teeth. 3Developmentally and histologically there are no funda-
mental divergences from the teeth of vertebrates lower in the
scale.

In mammals, as for example in the Dog, the teeth consist of
three kinds of tissue—the enamel, the dentine, and the cement.
The enamel is derived from the epidermis of the mouth cavity,
and the two remaining constituents from the underlying dermis.
The teeth originate quite independently of the jaws, with which
they are later so intimately connected ; the independence of
origin being one of the facts upon which the current theory

1 Of. Tomes, A. M'a.nual of Dental Anatomy, 5th ed. London, 1898.

h, Head ; t, great trochan-
ter ; £', tlxird trocbanter.
(From Flower's Osteology.)